save as a video as .mp4
The .blend file contains the 3D project.
To create an animation in a video format you need to render it first.
Select the output format and encoding, the name of the file and hit Animation (or CTRL F12

UPDATE:
One of the big changes in 2.79 is that they finally separated containers and codecs.
The encoding options from previous versions are still there, but now they have organized differently
If you select FFMpeg video, there will be a new tab for Encoding options.

There you can select the container and the codec for the output file.
Container is the file format for the file, (Mov, Avi, Ogg, MP4, to name a few) sometimes is referred as a &quot;wrapper&quot;.
Codec (short for Compression-Decompression) Is how the image is compressed. The picture quality, file size and processing power needed to view the file are largely determined by the kind of codec used and the settings used for it.